Mail processing systems for preparing mail pieces have long been well known and have enjoyed considerable commercial success. There are many different types of mail processing systems, including, for example, inserter systems that insert material into envelopes and mailing machines that print postage indicia on mail pieces. Optical sensors are commonly used in such mail processing systems to ensure that all parts in the system function in a coordinated way. For example, in a mail inserting system where a plurality of enclosure feeders are used to release documents onto a transport path and the released documents are collated into a stack for insertion into an envelope, optical sensors can be used to check the arrival of the envelope, the movement of the released documents, and so forth. Mailing machines range from relatively small units that handle only one mail piece at a time, to large, multi-functional units that can process thousands of mail pieces per hour in a continuous stream operation. The larger mailing machines often include different modules that automate the processes of producing mail pieces, each of which performs a different task on the mail piece. Such modules could include, for example, a singulating module, i.e., separating a stack of mail pieces such that the mail pieces are conveyed one at a time along a transport path, a moistening/sealing module, i.e., wetting and closing the glued flap of an envelope, a weighing module, and a metering module, i.e., applying evidence of postage to the mail piece. The exact configuration of the mailing machine is, of course, particular to the needs of the user.
There are generally two different types of optical sensors that are commonly used to track the location of each mail pieces as they pass through such mail processing systems. Each type includes a light-emitting diode (LED) and photo-detector. In one type, referred to as a through-beam detector, the LED and photo-detector are above and below the transport path and a mail piece passes between them. Detection occurs by the mail piece blocking the light from the LED that is received by the photo-detector. In the other type of sensor, referred to as a reflective sensor, the LED and photo-detector are located adjacent to one another in the same plane. As mail pieces pass over them, they are detected by the light from the LED that is reflected back to the photo-detector by the mail piece. Reflective sensors are generally used in areas where the transport path is not covered, e.g., the input hopper area, thereby not having any structure on which to mount the LED or photo-detector such that the mail piece can pass directly between the LED and photo-detector. Such reflective sensors must be sensitive enough to enough to detect the presence of mail materials of various degrees of optical reflectance.
Mail processing systems are subject to widely varying ambient light conditions, such as when positioned near an office window. In most situations the ambient light that leaks internal to the machine derives primarily, if not fully, from artificial lighting in the office. In other situations, however, the ambient light level can be much higher from additive sunlight. Ambient light can ‘leak’ into the system into the areas in which the photo-detectors are disposed (typically either along the deck or opposite it). This can cause problems, especially with respect to reflective sensors. The photo-detectors are typically continuously scanned in order to detect presence of a mail piece along the transport path, but varying ambient light conditions in some cases cause the sensor to signal to the mail processing system that a mail piece is present when in fact the photo-detector has detected only the elevated ambient light from office lighting or sunlight. In effect, such elevated ambient light saturates the photo-detectors making the controlling software think that a mail piece is reflecting light onto them, referred to as a false-detection. Such false detections made by sensors located in the input hopper area can result, for example, in the system randomly starting when no mail is actually present in the input area.
In such cases, end users may be forced to draw blinds in a room, reorient the system or place the system in an interior room to eliminate the high ambient light conditions from sunlight which trigger the false-detections noted above. But relying on end users to resolve a machine error may be problematic and limiting the placement of a mail processing system may not be practical for a particular installation. To help prevent such problems, reflective sensors are traditionally used with trade-offs. Either the sensors are sensitive enough to detect dark material (e.g., dark colors such as brown or black), but could be tricked by excessive levels of ambient light, or the sensors would be more immune to ambient light, but not reliably detect the presence of dark material. Accordingly, there is a need for a mail processing system having optical sensors that can detect dark material but function more reliably in varying lighting conditions.